1. Field of the Invention
This invention relates to a method and a system for shift control of an automatic transmission for a vehicle, and more particularly to improvements in a method and a system of shift control of an automatic transmission for a vehicle, wherein there are provided a first transmission section capable of automatically shifting gears in association with at least vehicle speed and engine load and a second transmission section capable of automatically shifting at least between the lower gear side and the higher gear side, and wherein the first transmission section and the second transmission section are simultaneously or alternately shifted, to thereby achieve multi-gear shifts.
2. Description of the Prior Art
As the FF (Front engine--Front drive) has been used more widely in the driving system of vehicles in recent years, there have occurred trends of high rpms and high output in an engine mounted on the vehicle. An automatic transmission of the FF system presents a disadvantage in that it is extremely difficult to increase the number of plates in a multi-plate clutch and in a multi-plate brake of a frictionally engaging device due to the restriction placed upon the total length of the automatic transmission.
In view of the above-described disadvantage, to improve the durability of the frictionally engaging device in the automatic transmission, there has been proposed an automatic transmission, wherein a design change is made such that the number of gears to be achieved may be increased by use of the existing automatic transmission added thereto with a so-called overdrive mechanism.
For example, in an automatic transmission, wherein an overdrive mechanism is added to a first transmission section of forward three gears, such a design change may be made such that the overdrive mechanism in the form of a second transmission section is shifted simultaneously or alternately with the first transmission section, as shown in FIG. 6, so that multi-gear shifts of six forward gears can be achieved.
However, with the above-described automatic transmission, wherein the first transmission section and the second transmission section are simultaneously or alternately shifted to achieve the multi-gear shifts, there has occurred a disadvantage in that there are gears achieved as the result of the first transmission section and the second transmission section being shifted in directions opposite to each other, and, unless the timing of these shift operations into these gears are controlled adequately and strictly, fluctuations in torque are high, whereby an uncomfortable feeling is experienced by the driver.
More specifically, as shown in FIG. 6, when the automatic transmission as a whole is shifted up from 2nd gear to 3rd gear and from 4th gear to 5th gear for example, the first transmission section is shifted to high gear, whereas the second transmission section is shifted to low gear. When the automatic transmission as a whole is shifted down from 5th gear to 4th gear and 3rd gear to 2nd gear, the first transmission section is shifted to low gear, whereas the second transmission section is shifted to high gear. From the above-described reasons, unless these shifts are effected accurately with adequate timing, the fluctuations in torque can be high, and moreover, in an extreme case, such a strange driving sensation is experienced that a down shift is performed immediately after an up shift or an up shift is performed immediately after a down shift.
On the other hand, similarly, to insure the durability of the frictionally engaging device, there are instances where the normal order of shifting, i.e. 1st gear-3rd gear-5th gear-6th gear in FIG. 6 in the automatic transmission added thereto with the overdrive mechanism is changed to the order of 1st gear-3rd gear-4th gear-6th gear, for example. According to this method, the former and the latter are equal in terms of number of gears, however, instead of the 3rd-5th gear shift, which is performed by the frictionally engaging device (so-called direct clutch) where the durability is most severely required, the overdrive mechanism (the second transmission section) is shifted, with 2nd gear of the first transmission section being left as it is, to achieve 4th gear, while the 4th-6th gear shift is performed with the shift point being slightly low, so that the durability of the frictionally engaging device can be ensured. Normally, the above-described shift is performed only in a region where a throttle opening (engine load) is high, whereas, in a region where the throttle opening is low, the order of shift of 1st gear-3rd gear-5th gear-6th gear is performed as in the past.
However, even in the above-described case, there occur gears which are achieved by shifting the first transmission section and the second transmission section in directions opposite each other as in the 4th-5th gear shift or the 5th-4th gear shift. As a consequence, the aforesaid disadvantages similarly occur.